Major pipelines, for transport of gases, liquids and slurries over long distances, are commonly made of steel which provides both strength and economy. Customarily, the pipelines are buried beneath the surface of the ground for protection, or pass under rivers or lakes in their paths. Under these conditions, such pipelines are subject to corrosion, due to electrochemical action, as a result of portions of the metal surface of the pipe coming into contact with electrolytes in the ground. To reduce the probability of corrosion occurring, these pipelines are commonly coated with an impermeable (electrically resistive) material, such as bitumens and mylar, etc.
Nevertheless, no coating is perfect, and all coatings deteriorate with time, aided by ground movement or as a result of attack by water, chemical or biological agents, resulting in ground contact points being developed. Such contact points are generally known as "holidays" in the pipeline industry. It is at such holidays that corrosion may occur.
Metal ions escape from the pipe at the ground contact point when the metal of the pipe is electropositive in potential relative to the electrolyte with which it is in contact in the ground. This causes corrosion. In the extreme, the pipe is so weakened by the loss of metal that a leak is formed, and the contents being transported start to escape. To reduce the possibility of such a condition occurring along the pipeline, corrosion engineers take steps to lower the potential of the pipe relative to the ground, by applying a negative DC voltage, using a DC generator. This is the practice of "cathodic protection" or "CP".
Further details of cathodic protection practices may be found in such sources as:
1) Parker, Marshal, E.; Pipeline Corrosion and Cathodic Protection: a Practical Manual for Corrosion Engineers. 3.sup.rd Edition. Gulf Publishing Co., Houston, Tex. 1984, or PA1 2) Morgan, John, H. Cathodic Protection (Second Edition), National Association of Corrosion Engineers. Houston, Tex., 1987.